1. Field of the Invention
The present invention relates to a high output-power semiconductor laser device having a sufficiently low noise characteristic and having sufficiently high resistance to the returning light of an emitted laser beam. The laser device is applicable as a light source for a recording/reproducing type magneto-optical disk or the like. More particularly, the present invention relates to a high output-power semiconductor laser device which utilizes a saturable absorber to induce a self-sustained pulsation phenomenon for obtaining a sufficiently low noise characteristic, and a method for producing the same.
2. Description of the Related Art
A semiconductor laser device can be used as a light source, incorporated in an optical disk apparatus, for reading signals. In such an optical disk apparatus, the emitted laser beam from the semiconductor laser device may be reflected on a plane of the disk back into the semiconductor laser device. In such a case, the semiconductor laser device tends to experience an extremely large amount of noise (i.e., a so-called "noise by returning light") resulting in an incorrect reading of the signals from the optical disk.
In order to minimize the noise by returning light of the semiconductor laser device, a self-sustained pulsation phenomenon is commonly used. In a self-sustained pulsation type semiconductor laser device utilizing the self-sustained pulsation phenomenon, the coherency of the laser beam is decreased by a self-sustained pulsation. Therefore, even when the laser beam is reflected from the plane of the disk back into the semiconductor laser device, no interference is generated inside the semiconductor laser device. As a result, a substantially low noise characteristic of the semiconductor laser device is ensured. In theory, the self-sustained pulsation is induced by providing a saturable absorber in an active layer of the semiconductor laser device (M. Yamada: IEEE J. Quantum Electron., vol. QE-29, No. 5 (May, 1993), pp. 1330-1336).
FIG. 12 is a cross-sectional view showing a conventional semiconductor laser device 50 in which a low-noise operation is achieved by self-sustained pulsation (see the proceedings of the 14th IEEE International Semiconductor Laser Conference, Th4.1 (1994), pp. 247-248).
In this conventional laser device 50, an n-GaAs buffer layer 52, an n-al.sub.x Ga.sub.1-x As (x=0.4) first cladding layer 53, an undoped multiple quantum well active layer 54 (hereinafter, simply referred to as an "MQW active layer"), a p-al.sub.x Ga.sub.1-x As (x=0.4) second cladding layer 55 and an n-Al.sub.x Ga.sub.1-x As (x=0.45) current blocking layer 56 are sequentially grown on an n-GaAs substrate 51 in a wafer state by a metal organic chemical vapor deposition method (hereinafter, simply referred to as an "MOCVD method"). Thereafter, the wafer is taken out from the MOCVD apparatus. A striped groove 57 (hereinafter, simply referred to as a "striped concave portion 57") is formed in the current blocking layer 56 such that the surface of the second cladding layer 55 is exposed. The striped concave portion 57 acts as a current path when the semiconductor laser device 50 operates.
Thereafter, the wafer is again placed in the MOCVD apparatus. A p-Al.sub.x Ga.sub.1-x As (x=0.4) third cladding layer 58 and a p-GaAs cap layer 59 are sequentially grown by the MOCVD method on the wafer so as to cover the current blocking layer 56 and the striped concave portion 57. After the third cladding layer 58 and the cap layer 59 have been grown, a p-side electrode 60 is formed on an upper surface of the cap layer 59 and an n-side electrode 61 is formed on a bottom surface of the substrate 51. The resulting wafer is then divided into a plurality of chips to obtain individual semiconductor laser devices 50.
Furthermore, in order to obtain a high output-power semiconductor laser device, the light emitting end facet is coated such that a reflectivity thereof is approximately 3%, and the other end facet is coated such that a reflectively thereof is approximately 94%.
When a voltage is applied between the electrodes 60 and 61 of the resulting semiconductor laser device 50, a current is injected into the active layer 54 via the striped concave portion 57, thereby generating laser oscillation. The oscillated laser beam is guided in the region immediately under the striped concave portion 57 due to the difference in the effective refractive index between the portion inside the striped concave portion 57 and the portions outside the striped concave portion 57.
In the conventional semiconductor laser device 50, when the thickness of the second cladding layer 55 is increased to a predetermined amount, and at the same time, the difference in the effective refractive index between the portion in the striped concave portion 57 and the portions outside the striped concave portion 57 is made substantially small, the light is not sufficiently confined in the striped concave portion 57. As a result, the width of the spread laser beam becomes wider than the width of the current injected region of the active layer 54. In such a state, portions of the active layer 54, corresponding to the outside of the striped concave portion 57, act as saturable absorbers for a self-sustained pulsation.
FIG. 13 is a cross-sectional view showing another type of conventional semiconductor laser device 70 in which a low-noise operation is realized by a self-sustained pulsation phenomenon. The semiconductor laser device 70 includes a saturable absorbing layer 76 separate from an active layer 74. A self-sustained pulsation is generated by a saturable absorbing function of the saturable absorbing layer 76 (see the proceedings of 12th Semiconductor Laser Symposium, March, 1995, p. 11).
Specifically, the semiconductor laser device 70 includes an n-GaAs buffer layer 72, an n-Al.sub.y Ga.sub.1-y As first cladding layer 73, an Al.sub.x Ga.sub.1-x As active layer 74 and a p-Al.sub.y Ga.sub.1-y As second cladding layer 75 sequentially provided on an n-GaAs substrate 71. A striped p-AlGaAs saturable absorbing layer 76 is provided on the second cladding layer 75.
A striped mesa region 79, including a p-Al.sub.y Ga.sub.1-y As third cladding layer 77 and a p-GaAs contact layer 78, is formed on the p-AlGaAs saturable absorbing layer 76. Furthermore, an n-AlGaAs current blocking layer 80 is provided on the second cladding layer 75 so as to sandwich the mesa region 79. A p-GaAs cap layer 81 is further provided over the striped mesa region 79 and the current blocking layer 80. A p-side electrode 82 is formed on an upper surface of the cap layer 81 and an n-side electrode 83 is formed on a bottom surface of the substrate 71.
In the case of the conventional semiconductor laser device 70, an Al mole fraction of the saturable absorbing layer 76 (i.e., a forbidden band width of the saturable absorbing layer 76) is made substantially equal to an Al mole fraction of the active layer 74 (i.e., a forbidden band width of the active layer 74), thereby ensuring the saturable absorbing characteristic of the device.
In the previously described semiconductor laser device 50, the portions of the active layer 54 corresponding to the outside of the striped concave portion 57 act as the saturable absorbers. In the semiconductor laser device 70, the saturable absorbing characteristic of the saturable absorbing layer 76, which is separately provided from the active layer 74, is used for the self-sustained pulsation.
In the semiconductor laser device 50, a sufficient saturable absorbing effect is required in order to realize a sufficiently strong self-sustained pulsation (i.e., a sufficiently low coherency). The following two techniques are available for this purpose.
(1) Increasing the amount of light that leaks into the saturable absorbers (i.e., the portions of the active layer 54 corresponding to the outside of the striped concave portion 57) by making the difference in the effective refractive index between the inside and the outside of the striped concave portion 57 sufficiently small; or
(2) Increasing the volume of each of the saturable absorbers by making the active layer 54 sufficiently thick.
However, the above-mentioned techniques (1) and (2) have the following drawbacks.
According to technique (1), the size of a light beam spot becomes wide as the amount of light that leaks into the saturable absorbers increases. Therefore, the radiation angle of the emitted laser beam with respect to a direction parallel to the active layer 54 becomes small. As a result, an undesirable change in the coupling efficiency of the semiconductor laser device 50 and a lens occurs upon comparison with the coupling efficiency obtained in the case of a semiconductor laser device with no self-sustained pulsation characteristic.
According to technique (2), the reliability of the semiconductor laser device in a high output-power condition generally deteriorates when the active layer 54 is made substantially thick.
In the case of the conventional semiconductor laser device 70, instead of using the active layer 74 provided outside the striped mesa region 79, the saturable absorbing layer 76 in the mesa region 79, provided separately from the active layer 74, is utilized for the self-sustained pulsation. Therefore, there is no need to expand the size of the light beam spot by increasing the amount of light that leaks into the active layer 74 provided outside the striped concave portion 79. Thus, in the semiconductor laser device 70, the aforementioned problem caused in the semiconductor laser device 50 does not occur.
However, in order to ensure a sufficiently strong self-sustained pulsation characteristic for a sufficiently low noise characteristic, the saturable absorbing layer 76 needs to be relatively thick. Since the saturable absorbing layer 76 is formed in the vicinity of the center portion of the oscillation region, a large amount of light is absorbed by the saturable absorbing layer 76 in an unsaturated state. Therefore, the operation current increases due to an increase in the oscillation starting current and deterioration of derivative efficiency. As a result, the operation characteristic of the semiconductor laser device 70 is deteriorated.